a medical procedure whereby a fiberoptic instrument is passed through the anus in order to thoroughly exam the large bowel and to some extent, the small bowel in its last few centimeters. This involes some prep work such as drinking a large volume of foul tasting fluid and receiving a sedative/narcoleptic drug so that the procedure is a painfree as possible. Frequently known by its actual name as colonoscopy.
Also, any unpleasant action in which you are on the shit end of the stick.

My doctorsent me in for a cornholeoscopy, lucky for me, no polyps.
My girlfriend took my creditcard and maxed it out, a real cornholeoscopy.